net/http: preserve nil values in Header.Clone

net/http: preserve nil values in Header.Clone

ReverseProxy makes a distinction between nil and zero-length header values.
Avoid losing nil-ness when cloning a request.

Thanks to Christian Mehlmauer for discovering this.

Fixes #53423
Fixes CVE-2022-32148

Change-Id: Ice369cdb4712e2d62e25bb881b080847aa4801f5
Reviewed-on: https://go-review.googlesource.com/c/go/+/412857
Reviewed-by: Ian Lance Taylor <iant@google.com>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>